1. Field of the Invention
The present invention relates generally to time division multiplex (TDM) digital communication systems, and more specifically to a digital receiver for receiving a TDM signal having a short preamble for clock timing recovery.
2. Description of the Related Art
In a conventional TDM communication system, digital signals from different data sources are encoded and sequentially transmitted on respective time slots, or channels. For carrier and clock timing recovery, each time slot contains a preamble. At the receive site, the TDM signal is demodulated into a baseband signal using the portion of the preamble for carrier recovery and the clock phase of each time slot is detected from the portion of the preamble for clock timing recovery. A clock recovery circuit is responsive to the clock phase of each successive time slot for generating clock pulses. By using the clock pulses, the encoded digital signal of each time slot is decoded.
However, since the clock timing of each data source is not necessarily synchronized with the clock timing of the other data sources, the preamble for clock timing recovery is required to be of sufficient length to accommodate such source-to-source clock timing differences. In addition, when noise is introduced in the immediately preceding time slot corrupting its clock phase information, the corrupted phase is used as an initial phase during the subsequent clock generation process. Therefore, the preamble of the conventional TDM communication system must be of sufficient length to ensure against such phase jitter, resulting in low transmission efficiency.